1. Field of the Invention
This invention relates to a porous membrane which consists essentially of a thermoplastic resin and has three-dimensional network structure including communicating micro-pores with a high porosity, and which is excellent in water entry pressure, air permeability and oil permeability and has good mechanical strengths and dimensional stability. It also relates a process for preparing such a thermoplastic resin porous membrane.
2. Description of the Prior Art
Recently, the technique concerning synthetic resin porous articles has made striking progress, and application ranges of porous articles having communicating pores as elements of various filters have especially be expanded. Among these porous articles, porous membranes including a water-repellent thermoplastic resin as a substrate have excellent functions suitable for microfilters for separating gases and separating oils and water from each other and for waterproof air-permeable membranes.
As the water-repellent porous articles, there have been known those prepared according to a sintering method, a nonwoven fabric method (a synthetic pulp method), a drawing method, a phase separation method and an extraction method.
According to the sintering method, a thermoplastic resin powder is partially melted at an appropriate temperature under an appropriate pressure to form a porous article. The porous article prepared according to this method has coarse pores, and the pore diameter is ordinarily scores of microns or larger. Accordingly, this porous article is not suitable for its separation of water from oil or other substances.
According to the nonwoven fabric method, it is ordinarily very difficult to obtain a porous article having micro-pores uniform in the pore diameter, and the porosity cannot be enhanced to the desired extent. Therefore, a porous article suitable for a microfilter cannot be obtained.
As the drawing method, Japanese Patent Publication No. 40,119/71 discloses a method comprising drawing a thermoplastic resin sheet of a molten state, annealing the resulting film and drawing the annealed film to form a porous membrane. The obtained product does not possess a three-dimensional network structure but a plane structure, and the porosity can be up to about 60% at most if it is required to form uniform and fine pores. Furthermore, since the drawing operation is conducted only in one direction, the anisotropy is conspicuous in the physical properties of the obtained porous article, and the strength factor F (described hereinafter) in the lateral direction is smaller than 1. The worst defect of the drawing method is that since the plastic material is highly drawn, the dimensional stability is poor and the use of the porous article at high temperatures is limited.
As the phase separation method, Japanese Laid-open Patent Application No. 22,398/80 discloses a method in which a thermoplastic resin is mixed with a high-boiling-point solvent inactive to said resin at a high temperature capable of providing a homogeneous phase to form a solution, the resulting solution is extruded through a slit and the extrudate is cooled to cause phase separation of the high-boiling-point solvent from the resin whereby a porous article is obtained. The porous article prepared according to the phase separation method comes to have a network structure in which cells having a relatively large size are communicated through fine pores. When the porosity is increased to a high level, the obtained porous article fails to have a strength sufficient to resist actual applications.
According to the extraction method, a thermoplastic resin is mixed with fine particles that can be extracted by an acid, an alkali or a solvent, optionally together with a liquid compound such as an oil or plasticizer, the mixture is molten, plasticized and extruded to form a film, and then, the fine particles and liquid compound are removed from the film by extraction using an appropriate solvent to form a porous membrane. In this method, in view of the moldability, it is not permissible to reduce the quantity of the polymer below a certain limit and at the extraction step, a considerable quality of shrinkage inevitable occurs. Accordingly, it is very difficult to obtain a porous membrane having a high porosity exceeding 80%. The porous membrane prepared according to this extraction method has a network structure, but the strength factor F (described hereinafter) is smaller than 1.
As another known drawing method, Japanese Patent Publication No. 18,991/76 discloses a method in which a pasty extrudate of polytetrafluoroethylene is monoaxially or biaxially drawn and the drawn extrudate is sintered at a temperature of higher than the melting point the polymer to obtain a porous article. According to this method, a fine-pore membrane having a network structure and a high porosity can be obtained, and the strength factor F (described hereinafter) of the porous article in the drawing direction is larger than 1 but the modulus factor F' (described hereinafter) is as small as 0.1 or less. Furthermore, the porous article prepared according to this method is characterized in that the creep under tensile and compressional stresses are very high, and it cannot practically be used without being supported by an other supporting member. The reason is that the porous membrane prepared according to this method has a structure formed of "nodes and fibrils". More specifically, the fibril is a fine fiber formed of very highly oriented PTFE (polytetrafluoroethylene) exhibiting a high tenacity, and the node is formed of a sintered body of unoriented PTFE particles. In this structure, the cohesive force among the particles is very low. Therefore, even when a very small stress is applied, slips are caused among the particles of the nodes. Such slip is a kind of plastic deformation, and even after the stress is removed, the original condition is not restored at all. This means that if the porous article is used under certain stress conditions, the creep is readily advanced and that even if the stress is removed, residual deformation is very large. Moreover, the resin that can be employed in this method is limited to a polytetrafluoroethylene resin.
As will be apparent from the foregoing description, a micro-pore membrane of a high porosity which is water-repellent and has a strength sufficient to resist actual applications cannot be provided according to the conventional teachniques of preparing porous membranes.